The purpose of this investigation is to extend previous studies which have indicated that the action of the UR enzyme in the glutamine synthetase cascade system may be mediated by a macromolecular inhibitor. Growth of E. coli W under various conditions demonstraed that the concentration of UR inhibitor cannot be manipulated with the various growth conditions employed. Dialyzed crude cell extracts inhibit UR activity in a concentration and time dependent manner. In addition, the inhibitory factor(s) in dialyzed crude cell extracts is resistant to short exposure to boiling followed by extensive dialysis.